The present invention relates to a perpendicular magnetic recording medium in which mass information can be recorded.
In a longitudinal magnetic recording system used in current magnetic disk storage equipment, it is necessary to increase the coercivity of the recording layer and decrease film thickness in order to increase the linear recording density, because the magnetization recorded in the medium is adjoined in mutually opposite directions. However, a problem arises that the write magnetic field of the recording head becomes insufficient when the coercivity of the recording layer is increased, and that recorded information may be lost due to thermal fluctuations when the film thickness of the recording layer is decreased. Because of problems like these, improving the recording density using a longitudinal magnetic recording system becomes difficult. Attention has been paid to a perpendicular magnetic recording system in order to solve these problems. A perpendicular magnetic recording system is a system of forming recorded bits so that a recording medium will be magnetized perpendicularly to the surface of the recording medium and the magnetization directions in adjacent recorded bits will be anti-parallel to each other. Since the demagnetization field in the magnetic transition region is small compared to a longitudinal recording system, the medium noise can be reduced and the recording magnetization can be stably maintained during high density recording.
As a magnetic recording layer (recording layer and magnetic layer) of the perpendicular magnetic recording medium, Co—Cr—Pt-based alloy films, which are also used for a longitudinal magnetic recording medium, have been studied. Reduction of the crystal grain size in the recording layer, reduction of the grain size distribution, and reduction of inter-granular exchange coupling are important for reducing the noise and improving the thermal stability of the medium using these recording layers. In the case when the Co—Cr—Pt-based alloy films are used for the magnetic recording layer, the c-axis of the hexagonal close-packed (hcp) structure is aligned all together in a direction perpendicular to the film surface, and only a slight difference of the in-plane crystallographic orientation exists in the adjoining crystal grains. Because of this, Cr segregation to the grain boundaries hardly occurs. This results in insufficient magnetic decoupling of crystal grains and/or an increase in crystal grain size due to coalescence of crystal grains in the course of crystal growth. Consequently, it is difficult to obtain low noise characteristics.
In order to solve problems like these, a granular medium has been proposed in which crystal grains are surrounded by non-magnetic compounds such as oxides and nitrides. For instance, JP-A No. 342908/2002 discloses a manufacturing process in which a recording layer mainly composed of a CoCrPt-based alloy includes Si oxides and the Si content, converted to Si atoms, is 8 at. % or more and 16 at. % or less, and the recording layer is deposited in a chamber under an Ar gas pressure of 0.133 Pa or more and 2.66 Pa or less by sputtering. Because of this, it is understood that inter-granular exchange coupling of CoCrPt crystal grains can be reduced, and high coercivity and SNR can be obtained. Moreover, in IEEE Transactions on Magnetics, Vol. 40, No. 4, July 2004, pp. 2498-2500, “Role of Oxygen Incorporation in Co—Cr—Pt—Si—O Perpendicular Magnetic Recording Media”, a process for manufacturing a recording layer having a granular structure is disclosed in which a composite target containing a Co—Cr—Pt based alloy and SiO2 is used and DC magnetron sputtering is used in an argon-oxygen mixed gas atmosphere. It is reported that performing reactive sputtering in an oxygen-containing atmosphere not only increases the coercivity but also improves the recording performances. Moreover, it is reported that the appropriate oxygen partial pressure is decided depending on the SiO2 concentration, and the appropriate oxygen partial pressure becomes higher with lower SiO2 concentration, and that the magnetic properties and recording performances deteriorate drastically when the oxygen concentration exceeds the appropriate value and reaches an excessive condition. In the case when reactive sputtering is applied under an oxygen-containing atmosphere, obtaining homogeneous magnetic properties and recording performance across the entire disk surface become very difficult because the oxygen concentration taken up into the magnetic film changes depending on the position on the disk, caused by the inhomogeneity of the distribution of oxygen concentration in the sputtering chamber.